wing wall

noun2

1. •
   In a bridge, a retaining wall on either side of an abutment.

   • Such an abutment would typically have wing walls that have sloping tops, to match the profile of the embankment. The wing walls are usually angled back from the abutment wall. — 2023
2. •
   With a tunnel, a retaining wall on either side of the portal.

   • The later tunnel has a portal of blue engineering bricks, and the curved wing walls are wider than those of the old tunnel, which has a rather cramped appearance. — 1953